Sync problem, IllegalArgumentException

I bought this app two days ago but it isn't able to sync my owncloud calendars. I used Owncloud 8.0 (on private webspace) and the error appeared. I decided to upgrade to the latest Owncloud version 9.1.4, but still same problem. Some exception is always risen inside Davdroid. The stack trace looks like this is a problem only for a specific event, but I'm not sure.